docs(multichannel): make channels_as_legend example informative

Default composite gave every channel the same legend colour. Subset to
three channels with an additive palette so the legend maps to what is
drawn; note why white-based cmaps wash the composite out.

Pin squidpy>=1.8.2 (first release with datasets.cells).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011JMbY8asahxaytthjvCKDv

Author: timtreis

pyproject.toml

@@ -53,7 +53,7 @@ watermark = "*"
 
 [tool.pixi.pypi-dependencies]
 spatialdata-plot = { version = ">=0.4.1", extras = ["interactive"] }
-squidpy = "*"
+squidpy = ">=1.8.2"
 
 [tool.pixi.tasks]
 kernel-install = 'python -m ipykernel install --user --name sdata-plot-tutorials --display-name "sdata-plot-tutorials"'